Aircraft stiffening section



`lune 18, 1968 F. J. LOWES 3,388,522

AIRCRAFT STIFFENING SECTION Filed Oct. 1l, 1965 2 Sheets-Sheet l INVENTOR.

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June 18, 1968 F. J. Lowes AIRCRAFT STIFFENING SECTION 2 Sheets-Sheet 2Filed Oct. ll, 1965 kmll O kmw INVENTOR.

den'c cf Lowes United States Patent O 3,388,522 AIRCRAFT STHPFENENGSECTEN Frederick l?. Lowes, Midland, Mich., assigner to The Dow ChemicalCompany, Midland, Mich., a corporation of Delaware Snbstituted forabandoned application Ser. No. 172,666, Feb. 12, 1962. This applicationct. l1, 1965, Ser. No. 567,597

3 Claims. (Cl. S2-618) ABSTRACT UF THE DSCLGSURE This invention concernsstiffening sheet members having no straight lines of weakness and whichcan take varying cross-sectional forms. For example, such a sheet membercan form the core of an air foil structure.

This application is a substitute for non-copending application Ser. No.172,666, filed Feb. 12, 1962, now abandoned.

This invention relates to a method and means for forming a sheet ofmaterial for use as a stiliening member in a composite structure, aswell as to the stitening member formed by the method of the invention.

The rapid increase being made in aircraft and missile velocities hasrequired greater strength air foils, and at the same time, has requiredthat such air foils be made thinner.

The current practice utilized in the fabrication of high speed (Mach 2or greater) aircraft wing sections, involves two general practices.

The first is to make the wing section in two parts, generally split onthe mean cord, or neutral axis. The parts are forged from a given alloyto a very close approximation of the required dimensions, then machinedso that desired tolerances are attained. To lighten the section, theback is sunk by either conventional, or chemical milling. The twosections, lower and upper contour, are then joined by a number ofconventional techniques.

The second technique, which is most frequently used when fabricatinglarge surfaces, is the honeycomb type of construction. Such constructioninvolves the manufacture of a metallic hexagonal honeycomb sheet ofsuitable alloy joined by line welding, or polymeric adhesive. Thehoneycomb sheet is then milled, in a rather diicult and cumbersomemanner, to form the desired wing contour, following which metallic skinsare applied to the edge surfaces by either organic adhesive, or brazing.

When using organic adhesives, a considerable amount must be used toassure adequate bonding, thus unnecessarily increasing the weight of thecompleted air foil. On the other hand, brazing techniques generallyrequire an inert atmosphere, and surplus brazing material alsounnecessarily contributes to the Weight of the air foil.

An air `foil made in accordance with the principles of the presentinvention, will be found to provide a solution to the problem ofobtaining high strength and thinner sections in high speed aircraft wingstructure.

Yet another object of the invention is to provide an air foil stitfenin7 member having no straight lines of weakness.

Brielly, the inventive concept involves the formation of a series ofsubstantially hyperbolic paraboloidal areas in a sheet of material, Thevertical cross-sectional dimensions of the formed sheet are variable,thus making such formed sheets useful for specialized application, suchas in wing structures, i.e., air foils, used on aircraft. It will befound that the cost of a wing structure made in accordance with theprinciples of the present invention, will Cit 3,388,522 Patented June18, 1968 lCe be somewhere between the cost of the first and secondtechniques discussed above.

The main object of this invention is to provide a method and means forforming a sheet of material for use as `a stiffening member in acomposite structure.

A more specic object is to provide a method and means for the formationof a series of substantially hyperbolic paraboloidal areas in a sheet ofmaterial.

Another object of the invention is to provide a method and means for theformation of an air foil stiffening member, which is of medium cost, andpossesses high strength and low Weight, as compared with air foils madeby conventional methods.

Still another object of the invention is to provide a method and meansfor the formation of a series of substantially hyperbolic paraboloidalareas in a sheet of material, wherein the vertical cross-sectionaldimensions are variable.

A further object is to provide a stiffening member for use in an airfoil, which member has a plurality of indentations arranged so that thevertical distance between the air foil surfaces are of variabledimension.

These and further objects and features of the invention will become moreapparent from the following description and accompanying drawingswherein:

FIG. 1 is a side view of a peg die arrangement used in the practice ofthe invention;

FIG. 2 is the same but showing a sheet of material in process offormation;

FIG. 3 is a section view taken through a sheet of material formed in thepeg die arrangement of FIG. l;

FIG. 4 is a section view illustrating an assemblage of two sheets of thetype shown in FIG. 3;

FIG. 5 is a side view similar to FIG. 2, but showing a modified type ofpeg die arrangement;

FIG. 6 is a partial section view of a stitfener section made from asheet of material formed in a peg .die arrangement ofthe invention;

FIG. 7 is a plan view illustrating the relative position of variousrods-when the supporting die plates are brought together-in a first typeof peg die arrangement;

FIG. 8 is a section view through a sheet of material when formed in thepeg die arrangement of FIG. 7, and as seen along line 8 8 in FIG. 7;

FIG. 9 is a plan view illustrating the relative position of variousrods-when the supporting die plates are brought togetherin a second typeof peg die arrangement; and

FIG. 10 is a section view through a sheet of material when formed in thepeg die arrangement of FIG. 9, and as seen along line 10-10 in FIG. 9.

Referring now to the drawings, and more particularly to FIGS. 1 and 2, apeg die arrangement is illustrated, which includes a lower die portion16, and an upper die portion 18. The lower die portion is formed of aflat plate 20, and a plurality of pegs, or rods 22, of equal height,secured normal to the plate in accordance with a given repetitivepattern. The upper die portion is formed of a flat plate 24, and aplurality of pegs, or rods 26, of variable height secured normal to theplate in accordance with a given repetitive pattern. The variable heightof the pegs 26 will generate a curve if all end points thereof arejoined, such curve simulating the contour of one side of an air foil. Itwill be understood, that the pegs 22 and 26, in addition to extending agiven distance across the width of their respective plate, also extend agiven distance along the length of their respective plate to provide thedesired air foil taper along the length thereof. The pegs 22 and 26, arearranged to intertit in predetermined lateral spaced relationship, whenthe die portions 16 and 24, are moved toward each other.

A flat sheet of ductile material 28, such as aluminum, magnesium, andtheir alloys, stainless steel, etc., which is to be formed, or operatedupon, is placed in the peg die arrangement so that it rests upon thepegs 22. Relative movement is effected between the die portions 16 and1S, which causes a plurality of indentations to be formed in the sheet,as illustrated in cross-section in FIG. 3. The forming step may becompleted in one operation, hot or cold, or annealed and drawn insuccessive steps.

It will be understood, that the peg die arrangements i6 and 18, areplaced in suitable press equipment (not shown) such as a double actiondraw press where a pressure plate is utilized, or in a hydraulic presswhere the draw rate may be controlled. In the latter case, a draw ringmay be advantageously used.

After the formed sheet 30 is trimmed, it may be joined to a similarlyformed and trimmed sheet 32 by means, such as spot welding, at points ofContact, more particularly at the flat surface areas formed by the endsof the pegs 22. The resulting lassemblage is then covered by a metalskin 34 which is secured by spot welding, or adhesively bonded to thesheets 3G and 32 to form the completed surface. An air foil made in suchmanner will be found to be much stronger than those formed by moreconventional methods, one reason being the bonding of ats of greatersurface as compared to the relatively smaller bonding surfaces as foundin comparable structures. Also, because there are a joined series ofsubstantially hyperbolic paraboloidal areas over the entire area of eachsheet 30 and 32, no straight lines of weakness exist within the sheets.

A modified form of peg die arrangement is illustrated in FIG. 5, whichdiffers from that described above chiefly in the fact that theindentations formed by the pegs, generate points of a curve on bothsides of the material upon drawing, which is equivalent in form to theexterior surfaces of the composite air foil shown in FIG 4. Toward thisend, a plurality of pegs, or rods 36, atiixed to a lower die plate 38,have the ends of the pegs at variable heights to generate the desiredcurve, while a plurality of pegs, or rods 40, affixed to an upper dieplate 42, have the ends of the pegs arranged at variable heights togenerate a similar curve.

It will be seen, that a sheet of material 44, formed in the peg diearrangement of FIG. 5, will have sets of indentations arranged toprovide a stitiening section for an air foil having both skin surfacescurved uniformly from the lead edge thereof.

It may be mentioned that the lateral spacing of the pegs, need not beuniform. That is in forming a sheet for use in an application havingareas of high stress, such as at the thin ends of a section, it may bedesirable to place the pegs of the forming die closer together in theregion of high stress to provide more material in a transverse(vertical) direction so as to accommodate the higher stress'.

Other modifications of formed sheets may be advantageously made in pegdie arrangements embodying the principles of the invention. For example,an air foil assemblage 46 shown in FIG. 6, including a stiffener section48, may be used as a stiffening layer in a conventional wing structurehaving ribs and spars. Thus, viewing FIG. 6, for example, a bottom skin47 would actually be the upper skin of a conventional wing on which thestitfener 48 is attached and which is in turn covered by another skin 49on the top thereof.

As has been mentioned, the arrangement of the pegs may be varied.Consider the arrangement illustrated in FIG. 7, which illustrates howthe pegs of a lower and an upper die arrangement intertit when movedtoward each other. It will be seen that the pegs of the lower die, shownin cross-hatching, will provide a rectangular patlil tern relative tothe pegs of the upper die. A tiat plate 50, operated upon in the peg diearrangement of FIG. 7, will have the cross-section shown in FIG. 8. InFIG. 9, the pegs of the lower die, will provide a hexagonal patternrelative to the pegs of the upper die. A flat plate 52, operated upon inthe peg die arrangement shown in FIG. 9, will have the cross-sectionshown in FIG. 10. Accordingly, the spacing of the pegs in the FIG. 7arrangement will lead to a geodetic weld pattern, whereas the spacing ofthe pegs in the FIG. 9 arrangement will give a linear pattern, whichunder certain conditions, is easier to fabricate. In either case, thearea of a formed sheet between each four pegs of one die portion,approximates a hyperbolic paraboloid, having the equation It will beobvious that a sheet formed in the manner as disclosed above, may findapplication in other than air foil structures. For example, formedsheets having varying cross-sections formed in accordance with theprinciples of the invention, could find use as reinforcements in unusualarchitectural structures, automotive body parts, and the like.Furthermore, the sheet material being used may be of differentthickness-such as by welding or bonding sheets together-before drawingoperations, to provide greater strength in certain areas, and inaddition to being metal, may be in plastic, or bre reinforced plasticsheets.

The foregoing description has been given in detail without thought oflimitation since the inventive principles involved are capable ofassuming other forms without departing from the spirit of the inventionor the scope of the following claims.

Accordingly, what is claimed as new is:

l. An air foil structure including a pair of stitfening members, eachmember comprising a sheet of rigid material having joined series ofsubstantially hyperbolic parabeloidal areas over the entire area of saidsheet, one side of said sheet having a surface which lies substantiallyin a flat plane while the other side of said sheet lies in a curvedplane, the shape of one sheet approximating the curve of one surface ofsaid air foil, and the shape of the other sheet approximating the curveof the other surface of said air foil, and said pair being joinedtogether at their flat plane surfaces to form said air foil structure.

2. An air foil structure including a stiffening member comprising asheet of rigid material having joined series of substantially hyperbolicparaboloidal areas over the entire area of said sheet whereby nostraight lines of weakness exist within said sheet, one side of saidsheet having a surface which lies substantially in one plane while theother side of said sheet lies in another plane, the plane of at leastone of said sides being curved.

3. The air foil structure of claim 4 wherein the plane of the other ofsaid sides is substantially fiat.

References Cited UNITED STATES PATENTS 2,391,997 1/1946 Noble 244-1192,481,046 9/1949 Scurlo'ck ll-131 X 2,858,247 10/1958 De Swart 52--618 X2,978,006 4/1961 Clemens 161--127 X 2,988,959 6/1961 Pelkey et al.52-615 X 3,052,431 9/1962 Compton 244--1 17.1 3,151,712 lil/1964 Jackson52-615 3,227,598 1/1966 Robb 161--127 X 3,258,892 7/'1966 Rushton 52-615DAVID I. WILLIAMOWSKY, Primary Examiner. D. L. TAYLOR, Examiner.

